Create a rake task to validate state and enable feature flag

To enable epic work items, we need to ensure that the data is in sync. We know that there are mismatches that we aim to tackle in 17.6 and be fixed in 17.7. However these mismatches only affect ~1% of the instances.

To unblock 99% of our customers to enable epic work items earlier, we create a rake task that:

1. Checks if the data is in sync
2. Enables the feature flag

The rake task should not just validate the status of the sync, but also output which part of the data needs to be re-synced in the worst case. An example output could be:

rake our_new_rake_task

✅ Epics are in sync
⚠️ Hierarchy is not in sync
⚠️ Linked items are not in sync

Please execute the following to re-sync the data. Otherwise we cannot enable epic work items in 17.6.

- rake task 1
- rake task 2

To be decided: if we should allow to enable epic work items per root group or for the whole instance